1. Field of the Invention
The present invention relates generally to cellular radio communication systems, and more specifically to a technique for expanding the coverage by multicasting a downlink signal to a plurality of base stations.
2. Description of the Related Art
In a cellular radio communication system, the usual method of covering a large cell is to provide high power transmission from a high-tower antenna. However, the installation of such high-power base stations may not necessarily represent the best solution in remote fringe areas where the coverage does not fit their geographic contour. In addition, the transmission power of a base station must be limited so that the power of reception at users near the base station is not too strong. As an alternative, a plurality of base stations simply designed to serve as low-power relay stations are respectively installed in separate areas and a downlink signal from a central station is multicast on a single radio frequency from the base stations simultaneously. However, the transmission power levels of the base stations and their locations must be such that there are dead spots around each cell to avoid interference between the multicast signals. Although advantageous from the frequency savings point of view, the prior art solution is not appropriate for areas where traffic demand is high and cells must be clustered, allowing no dead spots.